The correct option is B where the weather is pleasant
An adverb clause is a group of words containing a subject and a verb,
and it functions as an adverb, by modifying an adjective, verb or an
adverb. Adverb clauses generally begin with a subordinate conjunction
(since, because, while, etc.)
Option B is correct because it is an
adverb clause of place, "where" refers to place and it has the
subject "the weather" and verb "is".
Option A is incorrect because it's a phrase, not a clause.
Option C is incorrect because it is not an adverb clause of place, "because" is an adverb of cause.
Option D is incorrect because the answer lies in B.